The hard X-ray response of epitaxial GaAs detectors

We report on hard X-ray measurements with two epitaxial GaAs detectors of active areas 2.22 mm2 and thicknesses 40 and 400 μm at the ESRF and HASYLAB synchrotron research facilities. The detectors were fabricated using high-purity material and in spite of an order of magnitude difference in depletion depths, they were found to have comparable performances with energy resolutions at −45°C of ∼1 keV fwhm at 7 keV rising to ∼2 keV fwhm at 200 keV and noise floors in the range 1–1.5 keV. At energies <30 keV, the energy resolution was dominated by leakage current and electromagnetic pick-up, while at the highest energies measured, the resolutions approach the expected Fano limit (e.g., ∼1 keV near 200 keV). Both detectors are remarkably linear, with average rms non-linearities of 0.2% over the energy range 10–60 keV, which, taken in conjunction with Monte-Carlo results indicate that charge collection efficiencies must be in excess of 98%. This is consistent with material science metrology which show that the material used to produce them is of extremely high purity with impurity concentration <1013 cm−3.